A record is made of the traffic every thirteenth day throughout the
year, and an average taken to determine its mean amount. Some such
general method of classifying traffic in units is desirable, as it
permits the traffic of a road to be expressed in one number.

Before this French method can be applied to the traffic of our country
it will be necessary to modify considerably the mode of rating. This,
however, is a matter which can be studied and properly adjusted by the
Office of Public Road Inquiries. It is most important to obtain a record
of the average number of horses and vehicles and kind of vehicles that
pass over an earth road in a day before the macadam road is built. The
small cost of such a record is trifling when compared with the cost of a
macadam road (from $4,000 to $10,000 per mile for a fifteen-foot road),
in view of the fact that an error in the selection of material may cost
a much larger sum of money. After a record of the traffic is obtained,
if the road is to be built of crushed rock for the first time, an
allowance for an immediate increase in traffic amounting at least to ten
or fifteen per cent had best be made, for the improved road generally
brings traffic from adjoining roads.

To simplify the matter somewhat, the different classes of traffic to
which roads are subjected may be divided into five groups, which may be
called city, urban, suburban, highway, and country road traffic,
respectively. City traffic is a traffic so great that no macadam road
can withstand it, and is such as exists on the business streets of large
cities. For such a traffic stone and wood blocks, asphalt, brick, or
some such materials are necessary. Urban traffic is such as exists on
city streets which are not subjected to continuous heavy teaming, but
which have to withstand very heavy wear, and need the hardest and
toughest macadam rock. Suburban traffic is such as is common in the
suburbs of a city and the main streets of country towns. Highway traffic
is a traffic equal to that of the main country roads. Country road
traffic is a traffic equal to that of the less frequented country roads.

The city traffic will not be considered here. For an urban traffic, the
hardest and toughest rock, or in other words, a rock of the highest
wearing quality that can be found, is best. For a suburban traffic the
best rock would be one of high toughness but of less hardness than one
for urban traffic. For highway traffic a rock o